Photolithography is a technique for producing images on semiconductor devices. Typically, an image formed on a mask or “reticle” is transferred to a semiconductor object, or wafer, where it exposes a resist layer placed on the object. It is desired to pattern smaller and smaller features on semiconductor objects, which requires the use of shorter and shorter wavelengths of the light that is used to image the patterns. The minimal printable feature size, referred to “Critical Dimension” is proportional to the wavelength of the radiation used by the printer and to a coefficient k1, and inversely proportional to the Numerical Aperture (NA) of the optics printer. Typical printing schemes provide k1 values of about 0.5-0.6.
Under these assumptions, decrease in wavelength to 193 nm or 157 nm enables imaging of patterns with resolutions of 110 nm and 90 nm, respectively. For further improvements in resolution, even shorter wavelengths are necessary.
Attempts to further decrease the optical wavelength to either Extreme UV (EUV or soft X-ray at wavelengths shorter than 20 nm) or to X-ray, as well as to use other sources such as electron beams, have so far been proven to be complex and expensive. It is, therefore, desirable to develop optical techniques capable of decreasing k1 values achieved in optical lithography. Such techniques are referred to as Resolution Enhancement Techniques or RET. The most commonly used RET methods consist of the use of Phase Shift Masks (PSM), use of Optical Proximity Correction (OPC), and use of oblique illumination. These techniques can provide for a minimum theoretical k1 value of 0.25 for a dense pattern. k1 values as small as 0.30 have been demonstrated.
Due to decreasing design rules and the wide use of RET the masks used in image-projection systems have become increasingly difficult and expensive to make. Since many masks are needed to form the multiple patterns required to manufacture an integrated circuit, the time delay in making the masks and the cost of the masks themselves is a significant cost in the manufacture of semiconductor devices. This is especially so in the case of smaller volume devices, where the cost of the masks cannot be amortized over a large number of devices. Thus, it is desirable to provide a high-throughput apparatus for making semiconductor chips while eliminating the need for expensive masks. It is also desirable to improve the obtainable resolution of optical lithography. Further, such a device may be useful for directly patterning a small number of objects, such as runs of prototype devices, and for making masks.
Various maskless lithography methods are known in the art. A first method is a stepper-like lithography method in which an entire continuous area of an image is printed simultaneously. Such a method is described by Tod Sandstrom and Niklas Eriksson in “Resolution Extensions in sigma 7000 imaging pattern generator” Proc. SPIE Vol. 4889, pp. 157-167 (2002). This method consists of a Spatial Light Modulator (SLM), which operates as a reflective, 2D programmable mask, and a DUV pulsed-laser source. The pattern is formed by stitching together from multiple SLM images where each sub-image is created with a single flash, while the stage carrying the wafer is moving continuously.
The SLM used in this architecture has continuous pixels (i.e. there is no substantial physical gap between adjacent pixels). Analog modulation, which is capable of creating gray-level modulation with negative amplitude (refers to as “Blacker than black”), is used. As a result, this technique allows for some RET techniques, such as Attenuated Phase Shift Mask. Nevertheless, the main drawback of this technique is that since it uses very small instantaneous field of views, high speed printing requires high speed of the stage. This limits the printing speed due to stage inaccuracies, and may also limit its possible use in conjunction with, immersion lithography. Also, since pulsed laser illumination is used in order to “freeze” the instantaneous Field of View location, multiple laser pulses cannot be used for noise reduction by averaging.
The second method includes imaging multiple spots that are distant from each other. Such a method is described at U.S. Pat. No. 6,133,986 of Johnson and U.S patent applications 20030123040 and 20030122091 of Almogy that are incorporated herein by reference. In this method adjacent spots are printed at different times. As a result, exposure signals are added incoherently, regardless of the phase and illumination method used to create a single spot. Since the use of RET requires spatial coherence, the latter method is not suitable for RET, and therefore has limited resolution.
Accordingly, it is desirable to develop an efficient high-resolution printer and high resolution method for printing patterns.